Megakaryopoiesis, the mechanism of blood platelet production, is difficult to assess in clinical practice as it relies solely on microscopic description. Recent studies described the presence of emergency megakaryopoiesis bypassing the classical maturation stages under certain conditions. Circulating megakaryocytes were identified in inflammatory and hemostatic conditions. The aim of this study is to characterize the phenotype of medullar and circulating megakaryocytes by flow cytometry during and after coronary artery bypass grafting.